The present invention relates to an ink-jet printer which performs printing by jetting ink onto a sheet of paper which is held on a rotary drum as a print medium.
High-performance and low-cost personal computers are easily available in recent years, and have come into wide use. In accordance with this, the demand for color printers is also increasing. A variety of ink-jet printers have been developed as personal-use color printers.
Conventionally, an ink-jet printer capable of printing 500 sheets or more successively is known. This ink-jet printer comprises a rotary drum which rotates at a constant circumferential speed, and a print head which jets color inks onto a sheet held on the peripheral surface of the rotary drum. The sheet is fed toward the rotary drum from the front side thereof, and printing is performed when the sheet is wound around the rotary drum. After printing, the sheet is removed from the rotary drum and discharged to the rear side of the rotary drum.
The print head is made up of nozzle units for yellow, cyan, magenta and black, which are arranged along the peripheral surface of the rotary drum. Each nozzle unit has a plurality of ink-jet nozzles which are arranged across the sheet in the main scanning direction parallel to the axis of the rotary drum) and jets ink with the rotation of the drum. Each nozzle unit is shifted in the main scanning direction at a constant rate each time the rotary drum makes one rotation, and returned to an original position after a predetermined number of rotations which cause the nozzle unit to be moved by a distance equal to the nozzle pitch. Each nozzle unit performs printing of the whole sheet by jetting in the main scanning direction and the sub-scanning direction perpendicular to the main scanning direction as described above. During this printing, the sheet is held on the rotary drum with electrostatic attraction, negative-pressure suction and mechanical clamping, taken singly or in combination.
The utilization of the electrostatic attraction is most advantageous in providing a small-sized ink-jet printer. Where the electrostatic attraction is utilized, a charger is provided to charge the sheet by applying electrostatic charges. The charger is formed to perform charging in a non-contact-manner that the charger does not contact the sheet, in a contact manner that the charger contacts the sheet, or in a combination of these manners. In general, interference with the sheet or the peripheral surface of the rotary drum can be avoided in the non-contact manner, although a high charging efficiency cannot be obtained since the charger indirectly charges the sheet with a use of the rotary drum. On the other hand, a high charging efficiency can be achieved in the contact manner since the charger is brought into contact with the sheet and mechanically presses the sheet against the rotary drum.
However, the contact manner requires measures for preventing the charger from interfering with the sheet and the peripheral surface of the rotary drum. In addition, the contact manner requires a sequence control of synchronizing the operation of the charger with the timing at which the sheet is fed to the rotary drum. Moreover, electrostatic charges are applied to the sheet's obverse side which is brought intd contact with the charger. This being so, if the sheet is relatively thick, the electrostatic charges do not serve to produce electrostatic attraction on the sheet's reverse side which contacts the rotary drum. As a result, the substantial charging efficiency is decreased. In each of the contact and non-contact manners, electrostatic attraction attenuates due to leakage of electrostatic charges which occurs upon ejection of ink while the sheet is rotated along with the rotary drum. This phenomenon leads to the jamming of sheets or the alignment error of color dots.